1. Field of the Invention
The present invention relates to an input/output section of a traveling-wave tube for amplifying microwaves. In particular, the present invention relates to the structure of a coaxial waveguide converter circuit for converting the mode of the microwave when a microwave is applied from a waveguide to an input coaxial section of a traveling-wave tube, or for converting the mode of the microwave when a microwave is delivered from an output coaxial section of the traveling-wave tube to the waveguide.
2. Description of the Related Art
FIG. 1 is a longitudinal sectional view schematically illustrating the configuration of a general traveling-wave tube disclosed in Japanese laid-open patent publication No. 2005-339892A. Traveling-wave tube 100 generally comprises electron gun 101, delay circuit 102, and collector 103. Delay circuit 102 comprises helix 105 securely supported by dielectric 106 within vacuum sheath 104. Delay circuit 102 comprises, at both ends thereof, input circuit 107 for applying a microwave to helix 105 within traveling-wave tube 100, and output circuit 108 for delivering a microwave which is amplified while it propagates through helix 105, respectively. When waveguides 109 are used in input circuit 107 and output circuit 108, a coaxial waveguide converter circuit is formed between waveguides 109 and input/output coaxial sections 110 of traveling-wave guide 100 for converting the mode of the microwave.
A structure as shown in Japanese utility model publication No. H02-32208 has been proposed for the coaxial waveguide converter circuit. As illustrated in FIG. 2, this structure comprises cylindrical coaxial outer conductor 203 which couples waveguide 201 with outer sheath 202 of a traveling-wave tube, and coaxial inner conductor 205 which extends within waveguide 201 along the center axis of coaxial outer conductor 203 from outer sheath 202 of the traveling-wave tube to connect helix 204 to waveguide 201. Furthermore, a gap between coaxial outer conductor 203 and coaxial inner conductor 205 is sealed by ceramic window 206 under vacuum. In addition, waveguide matching part 207 is used at the joint of coaxial inner conductor 205 and waveguide wall 201a for impedance matching of a coaxial section comprised of coaxial outer conductor 203 and coaxial inner conductor 205 with waveguide 201.
Waveguide matching part 207, which comprises a cylindrical member, is fitted into a hole formed through waveguide wall 201a from the outside of waveguide tube 201 for fixation therein, and cylindrical coaxial inner conductor 205 is fitted into waveguide matching part 207. A cylindrical hole of part 207 has its leading end portion narrower than the remaining portion, such that coaxial inner waveguide 205 is fitted into a narrow hole (hereinafter called “fitting hole 207a”) at the leading end of part 207. Also, part 207 is made of a resilient material (for example, phosphor bronze), and is formed with a plurality of slits 207b from the leading end thereof, as illustrated in FIGS. 3A and 3B.
Before such waveguide matching part 207 is fitted into waveguide 201 from the outside thereof, cantilever supports 207c, divided by slits 207b, are previously urged toward the center axis (in other words, fitting hole 207a is narrowed). By fitting coaxial inner conductor 205 into waveguide matching part 207 in this state, waveguide matching part 207 is brought into contact with coaxial inner conductor 205. The contact between waveguide matching part 207 and coaxial inner conductor 205 is maintained by the resiliency of cantilever support 207c. 
According to the structure of the above waveguide matching part 207, part 207 can be brought into contact with coaxial inner conductor 205 without requiring a high machining accuracy for part 207, and is also assembled into waveguide 201 with ease.
However, the waveguide matching part of the coaxial waveguide converter circuit as disclosed in Japanese utility model publication No. H02-32208 is configured to make a contact with the coaxial inner conductor by urging the cantilever support to narrow the coaxial inner conductor fitting hole. As such, when relying on manual operations, the fitting hole is non-uniformly narrowed, resulting in a non-circular fitting hole which is brought into contact with the cylindrical coaxial inner conductor. Consequently, the contact is exacerbated between the coaxial inner conductor and waveguide matching part. On the other hand, when the operation is automated to uniformly narrow the fitting hole, the manufacturing cost is increased.
On the other hand, the coaxial inner conductor fitting hole in the conventional waveguide matching part is a straight hole which has a diameter larger than that of the coaxial inner conductor. Specifically, as illustrated in FIG. 4(a), wall surfaces of cantilever supports 207c which define fitting hole 207a are substantially parallel with the center line of fitting hole 207a. Accordingly, when waveguide matching part 207 is brought into contact with coaxial inner conductor 205, with cantilever supports 207c being previously urged, the wall surfaces of cantilever supports 207c which define fitting hole 207a are inclined, causing fitting hole 207a to come into point contact with coaxial inner conductor 205, as illustrated in FIG. 4(b). This further engraves the problem of the contact when the fitting hole is manually narrowed.
As described above, when the coaxial inner conductor is insufficiently in contact with the waveguide matching part, a problem arises in which the heat dissipation capability from the coaxial inner conductor is reduced.
Specifically, in a traveling-wave tube, as an electron beam passes through the delay circuit, the electron beam impinges on the inner wall of the helix to generate heat. Heat is also generated due to a high frequency loss when a microwave passes through the helix. Such heat generated in the helix is dissipated from the outer sheath of the traveling-wave tube, and is also dissipated from the waveguide through the coaxial inner conductor and waveguide matching part connected to the helix, and the like.
However, when the heat dissipation capability from the coaxial inner conductor is reduced, this causes the temperature to rise in the coaxial section and helix, which results in degraded electric characteristics and instable operations. In the worst case, discharge, sputtering and the like have occasionally occurred in the coaxial section to render the traveling-wave guide defective in operation.
Also, since the temperature rises during the operation of the traveling-wave guide, the contact exacerbates between the coaxial inner conductor and waveguide matching part due to a difference in thermal expansion between respective parts which make up the coaxial waveguide converter circuit, possibly resulting in a further degradation of the heat dissipation effect from the coaxial inner conductor.